The word “daemon” actually comes from the Greek language, meaning an “inner or attendant spirit” (according to the Oxford American Dictionary).
It is a fitting name as a computer daemon is a continuous running program that does an action trigger only when it receives some input.
For example, a printer daemon sends all information when a user determines that they have to print a document. A daemon running in the mail server routes incoming mail to the appropriate mailboxes.
Web servers use an “HTTPD” daemon that sends data to users when they access web pages. While daemons were first used in the Unix operating system, they were later incorporated into Mac OS X, which is Unix-based.